The blimp will hover above the jet stream at an altitude of 65,000 feet for months at a time and will also have the ability to detect low-flying missiles that may have slipped underneath ground-based radars. Once operational, it will be an important early-detection element of the broader U.S. missile defense architecture. It may also add as a weather surveyor and telecom relay.There are a number of challenges associated with an effort of this nature.Solar cells and an advanced fuel cells that can deliver up to 500 kW must be developed to power the craft. An aerodynamic design and a control system must be developed to help keep the airship steady amid the high winds at that altitude, without consuming excessive power. Another important factor is determining how the airship would react to changing temperatures as the sun rises and sets every day, heating and cooling the helium. Then there's the major challenge of finding materials for the airship's skin that are capable of withstanding the extreme ultraviolet radiation at such high altitudes for extended periods without becoming brittle.

But this HAA is actually a little less ambitious than earlier designs. Before, the airship was supposed to be King Kong big, at 25 times the size of the Goodyear Blimp. Now, it's merely huge, at two-and-a-half Goodyears in length. Plans to power the airship with lasers seem to have also fallen by the wayside, for now.If everything goes well, a prototype HAA should be ready to fly in 2010. I can't wait.UPDATE 5:23PM: Via the Wonk, here's a presentation on "Advanced Concepts in Missile Defense." The HAA is in there, as well as a program for one interceptor with "multiple kill vehicles."